Principle analysis of various types of flowmeters
(1) Principles of mechanics: Instruments belonging to such principles have differential pressure and rotor type using Bernoulli's theorem; impulse type and movable tube type using momentum theorem; direct mass type using Newton's second law; The target type of the momentum principle; the turbine type using the angular momentum theorem; the vortex type using the principle of fluid oscillation, the vortex type; the pitot tube type using the total static pressure difference; the volumetric type, the enthalpy, the trough type, and the like.
(2) Electrical principle: The instruments used for such principles are electromagnetic, differential capacitive, inductive, strain resistant, etc.
(3) Acoustic principle: Ultrasonic type, acoustic type (shock wave type), etc. are used for flow measurement using the acoustic principle.
(4) Thermal principle: The heat, direct thermal, indirect calorimetry, etc., which measure the flow using the thermal principle.
(5) Optical principle: laser type, photoelectric type, etc. are instruments belonging to such principles.
(6) Originally based on physical principles: nuclear magnetic resonance, nuclear radiation, etc. are instruments of this type.
(7) Other principles: Marking principle (trace principle, NMR principle), related principles, etc.
Impulse flowmeter
The flowmeter weighing impulse flowmeter that measures the flow using the impulse theorem is used to measure the flow rate of the granular solid medium, and is also used to measure the flow rate of the mud, the crystalline liquid, and the abrasive. Flow measurement ranges from a few kilograms per hour to nearly 10,000 tons. The typical instrument is a horizontal force-separated impulse flowmeter. The measurement principle is that when the measured medium falls freely from a certain height h to a detection plate with a tilt angle, an impulse is generated, and the horizontal force component of the impulse is proportional to the mass flow rate. Measuring this horizontal component can reflect the mass flow.
Electromagnetic flowmeter
The electromagnetic flowmeter is made by applying the electric conductor to generate an induced electromotive force in a magnetic field, and the induced electromotive force is proportional to the flow rate, and is measured by measuring the electromotive force to reflect the flow of the pipeline. Its measurement accuracy and sensitivity are high. Industrially used to measure the flow of water, slurry and other media. The maximum diameter can be measured up to 2m, and the pressure loss is extremely small. However, media with low conductivity, such as gas and steam, cannot be used.
The electromagnetic flowmeter has a high cost, and the signal is susceptible to external magnetic field interference, which affects the wide application in industrial tube flow measurement. To this end, the product is constantly improving and updating, to the development of computer.
Several problems of solutions
Recalculating the differential pressure scale
Temperature and pressure compensation can only reduce the measurement error, not only can not solve the problem fundamentally, but also the measurement signal exceeds 20mA, resulting in steam leakage measurement. The transmitter measurement signal exceeds 20 mA, indicating that the actual measured differential pressure signal ΔP exceeds the design differential pressure value.
Increase temperature and pressure compensation
When the temperature and pressure of the steam change, the density of the steam changes, and the steam flow measurement produces an error. Measurement error can be reduced by temperature and pressure compensation. Since the temperature of the saturated steam is a single-valued function of the pressure, the temperature and pressure compensation of the saturated steam can be pressure compensated or temperature compensated. Because the pressure signal detection is sensitive and the compensation accuracy is high, it is compensated by pressure and realized by DCS.
Conclusion
Steam is a special medium. As the pressure and temperature change, the density of steam changes. Therefore, it is necessary to compensate for temperature and pressure. When the pressure and temperature fluctuation of the steam are not large, that is, when the operating condition parameters deviate from the design parameters and the influence on the measurement is small, the temperature and pressure compensation measures can achieve the purpose of accurate measurement. However, when the operating parameters deviate too much from the design parameters or the operating parameters fluctuate frequently and are too large, even with the temperature and pressure compensation, it is difficult to meet the measurement accuracy requirements. At this point, only differential pressure or flow can be recalculated for a particular throttling element.